Observation of cavity polaritons in a metal-mirror Fabry-Pérot microcavity containing liquid-crystalline semiconductor based on perylene bisimide units.

We investigate the optical properties of a metal-mirror microcavity containing a liquid-crystalline (LC) perylene tetracarboxylic bisimide (PTCBI) derivative. Measurements of the transmission's incidence angle dependence show that the peaks are split in a complex way and shift as the angle changes. Further, measurements of the photoluminescence spectrum's emission angle dependence show that the peak also shifts with the angle, as in the transmission experiment. We also carry out a theoretical analysis; the theoretical and experimental results are in very good agreement, and we estimate the vacuum Rabi splitting energies to be about 212, 180, and 240 meV. In addition, the peak photoluminescence energy coincides with the lower polariton branch obtained by transmission experiment. Finally, in a time-resolved photoluminescence experiment, we observe a fast relaxation component that is not seen in the bare LC PTCBI film. We believe this is due to cavity effects increasing the spontaneous emission transition rate, indicating that the emissions are due to cavity polaritons.

Antiferroelectric Nature of CH3NH3PbI3-xClx Perovskite and Its Implication for Charge Separation in Perovskite Solar Cells.

Perovskite solar cells (PSCs) have been attracted scientific interest due to high performance. Some researchers have suggested anomalous behavior of PSCs to the polarizations due to the ion migration or ferroelectric behavior. Experimental results and theoretical calculations have suggested the possibility of ferroelectricity in organic-inorganic perovskite. However, still no studies have been concretely discarded the ferroelectric nature of perovskite absorbers in PSCs. Hysteresis of P-E (polarization-electric field) loops is an important evidence to confirm the ferroelectricity. In this study, P-E loop measurements, in-depth structural study, analyses of dielectric behavior and the phase transitions of CH3NH3PbI3-xClx perovskite were carried out and investigated. The results suggest that CH3NH3PbI3-xClx perovskite is in an antiferroelectric phase at room temperature. The antiferroelectric phase can be switched to ferroelectric phase by the poling treatment and exhibits ferroelectric-like hysteresis P-E loops and dielectric behavior around room temperature; namely, the perovskite can generate a ferroelectric polarization under PSCs operating conditions. Furthermore, we also discuss the implications of ferroelectric polarization on PSCs charge separation.

Nanoparticles speckled by ready-to-conjugate lanthanide complexes for multimodal imaging.

Multimodal and multifunctional contrast agents receive enormous attention in the biomedical imaging field. Such contrast agents are routinely prepared by the incorporation of organic molecules and inorganic nanoparticles (NPs) into host materials such as gold NPs, silica NPs, polymer NPs, and liposomes. Despite their non-cytotoxic nature, the large size of these NPs limits the in vivo distribution and clearance and inflames complex pharmacokinetics, which hinder the regulatory approval for clinical applications. Herein, we report a unique method that combines magnetic resonance imaging (MRI) and fluorescence imaging modalities together in nanoscale entities by the simple, direct and stable conjugation of novel biotinylated coordination complexes of gadolinium(III) to CdSe/ZnS quantum dots (QD) and terbium(III) to super paramagnetic iron oxide NPs (SPION) but without any host material. Subsequently, we evaluate the potentials of such lanthanide-speckled fluorescent-magnetic NPs for bioimaging at single-molecule, cell and in vivo levels. The simple preparation and small size make such fluorescent-magnetic NPs promising contrast agents for biomedical imaging.

Synthesis of [111]- and {010}-faceted anatase TiO2 nanocrystals from tri-titanate nanosheets and their photocatalytic and DSSC performances.

[111]- and {010}-faceted anatase nanocrystals with controllable crystal size and morphology were synthesized from tri-titanate H2Ti3O7 nanosheets by hydrothermal reaction. The nanostructures and the formation reaction mechanism of the obtained TiO2 nanocrystals were investigated using XRD, FE-SEM, and TEM. Furthermore, the photocatalytic and dye-sensitized solar cell (DSSC) performances of the synthesized anatase nanocrystals were also characterized. Two types of reactions occur in the formation process of the anatase nanocrystals. One is an in situ topochemical conversion reaction of the layered titanate structure to an anatase structure, and another is the dissolution-deposition reaction on the particle surface, which splits the formed nanosheet-like particles into small TiO2 nanocrystals. The surface photocatalytic activity and the DSSC performance of the anatase nanocrystals are dependent on the crystal facet exposed on the particle surface, which increases in the order of non-facet < [111]-facet < {010}-facet. The increasing order corresponds to the increasing order of the bandgap and energy level of the lowest valence band of the anatase nanocrystals. Furthermore, the facet of the anatase also affects the DSSC performance, which is enhanced in the order of non-facet < [111]-facet < {010}-facet.

Auger ionization beats photo-oxidation of semiconductor quantum dots: extended stability of single-molecule photoluminescence.

Despite the bright and tuneable photoluminescence (PL) of semiconductor quantum dots (QDs), the PL instability induced by Auger recombination and oxidation poses a major challenge in single-molecule applications of QDs. The incomplete information about Auger recombination and oxidation is an obstacle in the resolution of this challenge. Here, we report for the first time that Auger-ionized QDs beat self-sensitized oxidation and the non-digitized PL intensity loss. Although high-intensity photoactivation insistently induces PL blinking, the transient escape of QDs into the ultrafast Auger recombination cycle prevents generation of singlet oxygen ((1) O2 ) and preserves the PL intensity. By the detection of the NIR phosphorescence of (1) O2 and evaluation of the photostability of single QDs in aerobic, anaerobic, and (1) O2 scavenger-enriched environments, we disclose relations of Auger ionization and (1) O2 -mediated oxidation to the PL stability of single QDs, which will be useful during the formulation of QD-based single-molecule imaging tools and single-photon devices.

Modification of TiO2 electrode with organic silane interposed layer for high-performance of dye-sensitized solar cells.

Back electron transfer from the TiO2 electrode surface to the electrolyte is the main reason behind the low-open circuit potential (Voc) and the low-fill factor (FF) of the dye-sensitized solar cells (DSSCs). Modifications to the TiO2 electrode, fabricated using {010}-faceted TiO2 nanoparticles with six different kinds of silane, are reported to decrease the back electron transfer on the TiO2 surface. The effect of alkyl chain length of hydrocarbon silanes and fluorocarbon silanes on adsorption parameters of surface coverage and adsorption constant, interfacial resistance, and photovoltaic performances were investigated. Adsorption isotherms, impedance analysis, and photovoltaic measurements were used as the investigation techniques. The reduction of back electron transfer depended on the TiO2 surface coverage by silane, alkyl chain length, and the molecular structure of the silane. Even though Voc and FF were improved, significant reduction in short-circuit photocurrent density (Jsc) was observed after silanization because of desorption of dye during silanization. A new approach, sequential adsorption process of silane and dye, was introduced to enhance Voc and FF without lowering Jsc. Heptadecafluorodecyl trimethoxy-silane showed the highest coverage on the surface of the TiO2 and had the highest effect on the performance improvement of the DSSC, where Voc, FF, and efficiency (η) were improved by 22, 8.0, and 22%, respectively.

Single-molecule photochemical reactions of Auger-ionized quantum dots.

Photoinduced electron transfer in donor-acceptor systems composed of quantum dots (QDs) and electron donors or acceptors is a subject of considerable recent research interest due to the potential applications of such systems in both solar energy harvesting and degradation of organic pollutants. Herein, we employed single-molecule imaging and spectroscopy techniques for the detection of photochemical reactions between 1,4-diaminobutane (DAB) and CdSe/ZnS single QDs. We investigated the reactions by analyzing photoluminescence (PL) intensity and lifetime of QDs at ensemble and single-molecule levels. While DAB was applied to single QDs tethered on a cover slip or QDs dispersed in a solution, PL intensity of QD continuously decreased with a concomitant increase in the PL lifetime. Interestingly, these changes in the PL properties of QD were predominant under high-intensity photoactivation. We hypothesize that the above changes in the PL properties surface due to the transfer of an electron from DAB to Auger-ionized QD followed by elimination of a proton from DAB and the formation of a QD-DAB adduct. Thus, a continuous decrease in the PL intensity of QDs under high-intensity photoactivation is attributed to continuous photochemical reactions of DAB with single QDs and the formation of QD-(DAB)(n) adducts. We believe that detection and analysis of such photochemical reactions of single QDs with amines will be of considerable broad interest due to the significant impact of photoinduced electron transfer reactions in energy management and environmental remediation.

Blinking suppression in CdSe/ZnS single quantum dots by TiO2 nanoparticles.

The photoluminescence of semiconductor quantum dots and fluorescence of single molecules intermittently turn ON and OFF, a phenomenon referred to as blinking. In quantum dots, blinking occurs as a result of intermittent Auger ionization, which results in the formation of positively charged quantum dots. Due to strong Coulombic interactions, successive photoactivation of a charged quantum dot results in nonradiative carrier recombination, inducing long-lived OFF states in the intensity trajectories. Blinking is an undesirable property with respect to applications of quantum dots toward single-molecule imaging and single-photon logic devices. Here we report significant blinking suppression for CdSe/ZnS single quantum dots in the presence of TiO(2) nanoparticles. In this work, we continuously recorded photoluminescence intensity trajectories of single quantum dots with and without TiO(2) nanoparticles. Interestingly, the intensity trajectory of a single quantum dot that was covalently tethered on a cover glass and dipped in water resulted in near-complete blinking suppression as soon as a TiO(2) nanoparticle solution was introduced. The blinking suppression was associated with a decrease in the photoluminescence intensity but without considerable changes in the photoluminescence lifetime, indicating that nonradiative carrier recombination in quantum dots was channeled into electron transfer to TiO(2) nanoparticles and back electron transfer to quantum dots. On the basis of these experiments and recent reports on photoinduced electron transfer from quantum dots to TiO(2) nanoparticles, we hypothesize that blinking of a quantum dot can be suppressed by increasing the rate of nonradiative regeneration of its neutral state by interfacing with a well-defined charge carrier trap such as an electron acceptor, which accepts an electron during Auger ionization and neutralizes the charged quantum dot by back electron transfer. Correlation between blinking suppression and electron transfer in a quantum dot-TiO(2) nanoparticle system may have important implications, for the preparation of nonblinking quantum dot for incessant and on-demand light emission, donor-acceptor systems for efficient solar energy harvesting, and hybrid semiconductor materials for quantum optical devices.

Enhancement and suppression of terahertz emission by a Fabry-Perot cavity structure with a nonlinear optical crystal.

We have fabricated Fabry-Perot (FP) cavities in the THz region with a ZnTe crystal as a cavity layer by a simple stacking method. We observed more than a three times enhancement of the THz emission intensity in the FP cavities compared with the bare ZnTe crystal at the frequencies of the resonant modes and stopband edges. On the other hand, suppression of the THz emission occurs at frequencies in the stopband. The enhancement and suppression of the THz emission are caused by the modification of the optical density of state in the FP cavities compared to the vacuum.

Two-photon spectroscopy of core excitons in barium fluoride using synchrotron radiation and laser light.

We have conducted two-photon spectroscopy of core excitons in BaF(2). Synchrotron radiation and laser light were used for 5p core-electron excitation and Auger-free luminescence was detected as the signal. Two-photon excitation enables access to f and p orbitals that cannot be reached by one-photon excitation of electrons in p orbitals. It has been found that the spin-orbit splittings of 4f and 6p states of the Ba ion in BaF(2) are 0.7 +/- 0.1 and 1.4 +/- 0.1 eV, respectively.